Investigation of molecular structure and solvent/temperature effect on tautomerism in (E)-4,6-dibromo-3-methoxy-2-[(p-tolylimino)methyl] phenol, a new thermochromic Schiff base, by using XRD, FT-IR, UV–vis, NMR and DFT methods Gökhan Kaştaş a, ⁎, Çiğdem Albayrak Kaştaş b , Ahmet Tabak b a Samsun University, Facult of Aeronautics and Astronautics, Department of Aircraft Maintenance, Samsun, TurkeySamsun University, Faculty of Aeronautics and Astronautics, Department of Aircraft Maintenance, Samsun, Turkey b Sinop University, Faculty of Arts and Sciences, Department of Chemistry, Sinop, TurkeySinop University, Faculty of Arts and Sciences, Department of Chemistry, Sinop, Turkey abstract article info Article history: Received 2 February 2019 Received in revised form 28 April 2019 Accepted 26 May 2019 Available online 28 May 2019 The molecular structure and the solvent/temperature effect on the tautomerism in a new Schiff base, (E)-4,6- dibromo-3-methoxy-2-[(p-tolylimino)methyl]phenol, were investigated using spectroscopic (NMR, UV–vis, FT- IR), crystallographic (XRD), computational (DFT and TD-DFT) methods and harmonic oscillator model of aromaticity (HOMA). The XRD, DFT and FT-IR results show that the compound exists in the phenol-imine form in the solid state. HOMA indices support the aromatic structure of the compound. DFT calculations were performed to understand pro- ton transfer process and relatively close values were obtained for the energies of tautomers. UV–vis studies prove the solvent dependence of the tautomerism in the compound by revealing the existence of both phenol-imine and keto- amine forms in polar solvents and only the phenol-imine form in apolar solvent. The TD-DFT results for the electronic transitions lead to the same conclusion as the absorption spectra. 1 H NMR and 13 C NMR studies at room and low (-60 °C) temperatures indicate that the tautomeric equilibrium occurs rapidly in the compound. Therefore, it is difficult to observe two tautomers. However, the presence of tautomeric structures is clearly seen in acetone‑d 6 , alternatively underlying the solvent and temperature dependence of tautomerism in the title compound. © 2019 Elsevier B.V. All rights reserved. Keywords: Schiff Base Tautomerism Solvent effect Temperature effect Thermochromism Spectroscopy Computational methods 1. Introduction Many organic molecules that show reversible reaction between two different states when stimulated are proposed as an innovative material for data elaboration, storage and communication devices such as photo- chromic, fluorescence and multifunctional switches and memories, and used in studies on variable electrical current and ion transport through membranes [1–5]. On the basis of the technological applications, some of them are related to thermocromic and/or photochromic properties in solid form, o-hydroxy Schiff bases are considered to be an important class in the organic compounds. o-hydroxy Schiff bases show tautomer- ism in solid state and solvent media. As a result, these compounds can exist in two structures known as phenol-imine and keto-amine (Scheme 1), a special structural isomer that can be converted into each other [6–8]. The substituents affect the tautomeric structure in the solid state. While Schiff bases obtained from salicylaldehyde and its derivatives are generally found in phenol-imine form [9–12], it is observed that Schiff bases obtained from naphthaldehyde and those containing azo group pre- fer keto-amine form [13–17]. Studies on the tautomerism in solvent media show that proton transfer is also affected by the solvent type [18–22]. While it is possible to observe both forms in polar solvents, they are usually present only in phenol-imine form in nonpolar solvents [23,24]. To our knowledge, in the NMR studies of Schiff bases obtained from salicylaldehyde and its derivatives, the signals of both phenol- imine and keto-amine forms have not been clearly observed in solution [25]. 1 H NMR gives only average values for the proton resonance peaks in the case of a tautomeric equilibrium with a high rate, which makes im- possible to investigate the prototropic tautomerism [26]. In this study, the tautomeric equilibrium in a new Schiff base, (E)- 4,6-dibromo-3-methoxy-2-[(p-tolylimino)methyl]phenol, was investi- gated with spectroscopic methods such as X-ray diffraction (XRD), FT- IR, Ultraviolet-visible (UV–vis) and nuclear magnetic resonance (NMR) and with computational methods as density functional theory (DFT) and time-depended density functional theory (TD-DFT). We be- lieve that this study is one of nice examples of an integrated ap- proach to the investigation of solvent/temperature effect on tautomerism in o-hydroxy Schiff bases by combining the results of different spectroscopic tools. In addition, the fact that the tautomeric equilibrium could be characterized with NMR spectroscopy makes the current study one of rare examples when previous studies in lit- erature are considered. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 222 (2019) 117198 ⁎ Corresponding author. E-mail address: gokhan.kastas@samsun.edu.tr (G. Kaştaş). https://doi.org/10.1016/j.saa.2019.117198 1386-1425/© 2019 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa